A bar code reader typically uses a beam of light to read a bar code, which consists of alternating bars of differing reflectivities. The scanner then receives and interprets the fluctuations in the returning light that are caused by the bar code. It is known in the prior art to read bar codes by means of a hand-held wand which makes contact with the surface on which the bar code is printed. However, the need to make contact with the surface is frequently inconvenient and gives uninterpretable readings because the wand is not moved across the bar code with a sufficiently uniform velocity.
An alternative to a wand is a hand-held scanning reader which does not require physical contact with the bar code being read. A scanning reader typically produces a beam of light, called a scanning beam, which is scanned repetitively across a target object. If the scanning beam intercepts a bar code (or some other symbology), portions of it will be reflected back toward the scanning reader in a pattern corresponding to the pattern of the symbology encountered. This modulated light is detected by sensing circuitry in the scanning reader which in turn produces an electrical signal related to the returning light. The electrical signal is then analyzed to provide an indication of the relative widths of the bars and spaces of the bar code on the basis of relative time. That is, the widths of the alternating areas of different reflectivity are determined on the basis of the relative time duration of corresponding portions of the electrical signal, as related to the time for a single scan, also called the scanning time. This allows the scanning reader to be used with bar codes which have a wide variety of sizes, the important factor being that the relative widths of the elements of the bar codes be preserved. Accordingly, it is prferable that the scanning beam be scanned across the bar code at a substantially uniform rate in order to ease the task of interpreting the bar code.
In order to insure that the scanning beam is scanned at a substantially uniform rate, the scanning beam is typically reflected from a mirror within the scanning reader that moves in a repetitive pattern at a uniform rate. The mirror is generally driven by a small electrical motor under the control of electronic control circuitry. The mirror is typically either rotating at a constant speed or oscillating on the end of a shaft attached to a motor which can step between two extreme angular positions. Examples of rotating optical elements are shown in U.S. Pat. Nos. 4,025,761; 4,097,729; 4,450,350; 4,575,625; and 4,692,603. Examples of oscillating mirrors, also called dithering mirrors, are shown in U.S. Pat. Nos. 4,593,186; 4,736,095; and 4,808,804. In hand-held applications, a dithering mirror is generally preferable, since it can be made both lighter and more compactly than a rotating optical element.
The light source in a modem bar code scanner is generally a very low power solid state laser diode, since such devices are efficient and light, and can be made reliably and relatively inexpensively. Such devices may emit visible or invisible light. Where the light is visible, the scanning beam produces a visible line on the target object which helps the user to align the reader to a target object.
In many applications, the laser light is not visible to an operator. Thus, the operator is not able to determine if a beam of laser light is being emitted. Additionally, the user does not have any guide by which to align or focus the laser light. It is helpful to provide a visually discernible guide to indicate the existence and direction of the laser light to permit a user to "target" or "spot" the beam of laser light on a target object. Such beams are referred to as spotter beams.
It is known in the art to produce a spotter beam to aid the alignment of an invisible scanning beam with a target object. The spotter beam typically is a visible light beam directed toward the target object which presents a visible image near to, or superimposed upon, the scanning beam. The position of the visible image indicates to an operator the location of the invisible scanning beam's contact with the target object. The light source used for a spotter beam is typically a commercially available light-emitting diode (LED).
One known method of producing a spotter beam alternates the emission of the laser light with the visible light. This requires careful control of the timing of the laser diode and visible light LED so that the laser light is terminated during a short period of time during which the visible light LED is activated. In systems employing this method, the visible light emission is directed optically along the same scan path as the laser light. A collinear, visible light image and the invisible scanning beam are alternately shown on the target objects. Readers employing either of the above (i.e., visible scanning beam or invisible scanning beam combined with a visible spotter beam) typically produce a single, linear visible image for the user's reference. They do not provide any substantial indication of an appropriate focusing distance or parallel alignment of the scanning reader to the target object.
Another problem with such readers is that the dithering mirror stops at each angular extreme and returns over the same segment, causing the visible light from either the scanning beam or the spotter beam to dwell at the end of its travel and produce an undesirable bright spot at each end of the visible image.